Nuclear magnetic resonance (NMR) techniques have long been used to obtain spectroscopic information about substances, revealing the substance's chemical composition. More recently, spectroscopic imaging techniques have been developed which combine magnetic resonance imaging (MRI) techniques with NMR spectroscopic techniques, thus providing a spatial image of the chemical composition.
In recent years there has been increasing interest in the study of brain metabolism using proton MR spectroscopy and spectroscopic imaging because of its noninvasive assessment of regional biochemistry. While proton spectroscopy measures metabolite levels in a single volume, proton spectroscopic imaging (HSI) measures the spatial distribution of metabolites (e.g., N-acetylaspartate (NAA), total choline, total creatine, and lactate) over a predetermined volume of interest (VOI). HSI studies of the brain have shown locally altered metabolite levels in different pathologies, including brain tumors, multiple sclerosis, chronic and acute brain infraction, epilepsy and acquired immunodeficiency syndrome.
HSI requires spatial prelocalization of a volume of interest (VOI) to suppress overwhelming water and fat signals from superficial structures (bone, muscle, skin) which may lead to spectral artifacts in the region of interest. This prelocalization may be done by selective excitation of a rectangular volume using three-pulse localization schemes (e.g., using stimulated echo methods "STEAM"), by spatial suppression of outside volumes or by a combination of both. The degree of prelocalization with these methods is limited by imperfections in the field homogeneity of the radiofrequency coil, by intrinsic static magnetic field inhomogeneities in vivo, by relaxation processes and by limitations in gradient power. To improve the prelocalization a combination of selective excitation of the volume of interest and presaturation of outside volumes has been proposed. However, this approach is limited to the selection of rectangular volumes which do not follow the contours of organs such as the brain and is motion sensitive due to the application of strong gradient dephasing pulses for the STEAM localization sequence. To obtain a more flexible volume preselection a different technique has been proposed recently where selective excitation with a spin echo pulse sequence is used in one dimension and spatial presaturation of peripheral regions is used in the other dimensions. However, with this approach the degree of volumes prelocalization is limited, since spatial presaturation in vivo provides suppression factors of less than 100 (under favorable conditions) which leaves strong residual water and fat signals from peripheral regions. The technique has thus been used only at long echo times (272 ms) where water and fat signals strongly reduced as compared to metabolite signals due to their shorter transverse relaxation times and J-coupling. However, many metabolite signals also suffer strong signal losses at long echo times for the same reasons, and further, additional information is available at short echo times that is not present at long echo times. Concomitantly, most prior art localization techniques are not applicable to acquiring multiple volume data from nuclei that have a short T.sub.2 relaxation time, such as the phosphorus-31 (31P) nucleus or the sodium-23 (23Na) nucleus. Similarly, 31P and 23Na MRI and spectroscopic imaging provide additional information not found in HSI.
Thus, there is a need for improvements in techniques for prelocalizing a volume of interest, and preferably such improved techniques should render short echo time spectroscopic imaging practicable in order to elucidate additional spectral information which is not available at long echo times.
Another inherent requirement of spectroscopic imaging is that the pulse sequence for acquiring data encodes spectral information in addition to spatial information. This requirement presents additional problems and challenges in order to make spectroscopic imaging practicable. Some of the problems that must be addressed include: minimizing the deleterious coupling or overlap effects between the spectral encoding and the spatial encoding; minimizing the time required for acquiring the spectral and spatial information; increasing the signal-to-noise ratio; and increasing the spatial resolution of the spectral information. It is well recognized that many of these problems are related in a fundamental and/or practical manner, and thus, improving a given parameter may result in, or require, compromising another parameter. Indeed, however, one of the foremost limitations to clinical application of HSI is the length of time needed for data acquisition in order to provide images with sufficient spatial and spectral resolution. Particularly in the case of severely ill or instable patients, such study lengths are prohibitive.
Recent technical developments have sought to reduce the generally long acquisition times necessary for spectroscopic imaging. Three-dimensional phase encoding is desirable, since it yields complete volume coverage, permits thin slices and avoids chemical shift artifacts. However, phase encoding is very time consuming. Multislice techniques have been introduced as an alternative to reduce data acquisition times, but the number of slices with these techniques is limited due to the long data acquisition window. More recently, shorter acquisition times have been achieved by acquiring multiple individually phase encoded echoes during a single excitation. This method increases the signal-to-noise per unit time and unit volume, but introduces variable T2-weighting in k-space and is not compatible with short echo time acquisitions. Alternative approaches using fast imaging techniques with a Dixon-type echo time shifting to encode spectral information have been shown to be feasible, albeit at the expense of spectral resolution. In sum, these methods are not suitable for short echo time acquisitions.
Echo-planar spectroscopic imaging (EPSI), a much faster method proposed by Mansfield, (P. Mansfield, "Spatial Mapping of the Chemical Shift in NMR", J. of Physics D: Applied Physics, vol. 16 (1983), pp. L235-L238), and further developed by others, avoids these limitations by using a series of periodically inverted gradients to generate a train of echoes which contain both spatial and spectral information, thus permitting complete three-dimensional spatial encoding in a clinically reasonable time frame. EPSI, however, requires strong fast switching gradients with excellent eddy current performance. Further, due to the inherent convolution of the spatial and spectral information, spectral aliasing artifacts and localization constraints have precluded applications beyond initial feasibility studies.
Therefore, there is also a need for improved methods for acquiring spectroscopic data rapidly, without loss in spectral resolution or spatial localization.